The invention relates to a fuel preheating device for a heater, in particular a motor vehicle heater, operated with liquid fuel, with a burner that is connected to a fuel flow pipe containing a fuel filter for fuel supply and to a fuel return pipe for removal of excess fuel, and with a heat exchanger for a heat exchange between the hot combustion gases generated during combustion of the fuel supplied and a heat exchange medium flowing through the heat exchanger, and an exhaust gas discharge outlet through which the combustion gases, after passage through the heat exchanger, exit the heater.
From German Offenlegungsschrift 26 23 921 there is known, for example, a motor vehicle heater that is operated with liquid fuel of the type mentioned above. Here, a so-called "air heater" is involved in which a gaseous heat exchange medium, the so-called heating air, is used. Further, with this heater there is provided a heat exchanger for the cooling liquid of an internal combustion engine of a motor vehicle, so that the cooling liquid is warmed by the energy of the hot combustion gases generated in the heater during combustion, and the cooling liquid warmed this way is circulated in the cooling liquid circuit of an internal combustion engine.
Heaters are also known in which the burner has a spray nozzle or diffuser to spray or atomize the fed liquid fuel. To start such a heater, a thermostatically controlled nozzle assembly. preheating can be provided that preheats the liquid fuel, such as diesel fuel, in the nozzle assembly of the spray diffuser to assure starting of the heater even at low temperatures at which, for example, the paraffin tends to separate out of diesel fuel.
German Patent No. 33 08 891 discloses a fuel supply device for motor vehicle diesel engines in which an auxiliary heater that can be operated independently of the engine is provided in which the heat exchange medium circuit is connected to a heat exchanger in which the fuel to be fed to the internal combustion engine is preheated.
From German Offenlegungsschrift No. 37 21 834, a fuel preheating device for liquid fuel for heaters is known in which, to achieve trouble-free operation at low temperatures, the fuel is warmed, in a pipe section that runs between a fuel tank and a fuel pump, by a heat-transferring contact of this pipe section with a pipe conveying exhaust gas. Here, the heat exchange takes place in the area of the exhaust gas discharge connection of the heater, around which windings of the fuel pipe section are arranged. In such a design, the installation of the heater and its mounting are made more difficult and, further, here, no long-term, effective heat exchange is assured, since the contact areas tend to become contaminated by spray water and the like.
In all these known heaters, the tendency of paraffin to separate out of the fuel, especially diesel fuel, at low temperatures makes for difficulties, since the danger exists that the separated paraffin will clog filters in the fuel-conveying pipes. Previous fuel preheating devices in such heaters, such as thermostatically controlled nozzle assembly preheating devices, are electrically operated, and this electrical energy must normally be taken from the motor vehicle battery of the motor vehicle, consequently placing a greater load on the battery.